1. Field of the Invention
The present invention relates to an optical disc device which forms a recording mark having a different physical characteristic from that of the other portion, on a recording medium to read information from the optical disc on which information is recorded.
2. Description of the Related Art
Optical disc media include various media, such as a CD-R/RW, a DVD-RAM, a DVD±R/RW, a Blu-ray Disc (hereinafter referred to as a BD), and a HD DVD. Such optical disc media including one with two data layers have been widely used. What is termed as a DVD super multi-drive, which is provided for writing/reading on/from a CD-R/RW, a DVD-RAM and a DVD±R/RW, has widely available as the corresponding optical disc device. A high-performance drive, which is provided for a BD and a HD DVD, is considered to be widely available in the future.
Super resolution techniques and SIL (Solid Immersion Lens) for next-generation large-capacity optical discs are proposed, and one of the super resolution techniques is described in Japanese Patent Application Laid-open No. 2006-107588. Specifically, a phase-change recording film, which changes its optical characteristics when it melts, is buried in pits to conduct super resolution reading. In addition, recording marks are spatially separated from each other to reduce thermal interference between the recording marks and fluctuation of a super resolution region. The aforementioned configuration makes it possible to improve linear density and track density at the same time. As a result, storage capacity of the optical disc can be largely increased. Furthermore, light transmittance of the region between data pits is increased to allow an increase in an average light transmittance of a data surface. Thus, efficiency in light utilization is high, and this is advantageous for a multi-layering process. This type is hereinafter called as a three-dimensional pit selection type. As means for burying the recording film in the pits, it is possible to use a phase-change etching method (a method using a difference in etching speed between liquid crystal and amorphous) as described in Japanese Patent Application Laid-open No. 2005-100526, and a physical polishing method such as CMP (Chemical Mechanical Polishing).
When a super resolution optical disc is read, a super resolution signal is obtained from a mark smaller than optical resolution. Meanwhile, in a case where there is a mark larger than the optical resolution, a similar signal to that in a case of the normal optical disc is obtained. This signal is hereinafter called as a normal resolution signal. Thus, a readout signal of the super resolution optical disc results in such a nonlinear signal to which a super resolution signal and a normal resolution signal are added. Accordingly, instead of a general readout signal processing method, a nonlinear processing method needs to be carried out to obtain a good-bit error rate.
Japanese Patent No. 3781911 describes a nonlinear signal processing method in which a coefficient of equalization is changed according to a level of a readout signal. Furthermore, Japanese Patent Application Laid-open No. 2005-332453 describes a signal processing method in which crosstalk from adjacent tracks is suppressed using readout signals from three beam spots.